It is known that an axially magnetized linear force motor with an outwardly surfaced armature (hereinafter referred to generically as a linear force motor) linearly displaces the armature proportional to the magnitude of the driving current. The displacement of the armature of a linear force motor is linearly proportional to the magnitude of an input signal (for example a current input signal) supplied to the motor. The ratio of the displacement of the armature to the magnitude of the input signal is call the "gain" of the motor. Examples of linear force motors are generally disclosed in U.S. Pat. Nos. 4,235,153 and 4,127,835.
One difficulty with the linear force motor is that the gain can vary from motor to motor because part dimensions, magnet strengths, etc. vary from motor to motor. The variation of the gain is unacceptable for some applications.
The gain of a linear force motor can be controlled by machining the parts of the motor. However, setting the gain to a particular value by machining the parts requires assembling the motor, measuring the gain, disassembling the motor, and machining the parts repeatedly until the desired gain has been attained. This process is time consuming and adds to the manufacturing cost of the linear force motor.
The variations in gain between linear force motors can also be minimized by initially manufacturing the parts of the motors to exacting tolerances. However, the cost of a part is inversely proportional to the allowable variation of the part. Therefore, manufacturing the parts of the linear force motor to exacting tolerances will increase the cost of the linear force motor.